Powder metallurgy process for producing steel parts



Patented Nov. 29, 1949 POWDER METALLURGY PROCESS FOR PRODUCING STEELPARTS George Warren Webb, Revere, Mass., assignor to Isthmian Metals,Inc., Boston, Mass, a corporation of Massachusetts No Drawing.Application April 30, 1946, Serial No. 666,171

4 Claims.-

1 This invention deals with powder metallurgy; specifically it presentsan efficient method of eliminating deleterious oxygen from iron powdercompacts, especially in the presence of alloying elements.

The chief object of the invention is the making of iron and steelcompacts having superior physonly thus lose their effectiveness but theyactually become deleterious by being present as inclusions.

It is well known that iron powders may contain appreciable amounts ofsorbed material such as water and oxygen, that is material which isadsorbed or absorbed or chemically combined with the powder. Such sorbedmaterial may impair the physical properties of compacts made with suchpowder. This is especially the case when the process involves two ormore pressing operations with two or more heat-treatments.

I have found that the. presence of such sorbed material causesdifficulty in consistently obtaining uniformly high physical propertiesin the articles made from powdered starting material of apparently thesame composition andapparently identically processed.

When the starting material comprises iron powder containing sorbedmaterial to the extent of 0.6-0.8 (even though it includes nosubstantial amount of alloying agents having high afiinity for oxygen atthe sintering temperature) and such material is processed by priormethods in-c volving two pressings and two sinterings, the.

articles produced are not consistently uniform in their physicalproperties. Generally the physical properties are inferior as comparedwith results which are occasionally obtained. This variation is notexplained by variation in density because the densities of the articlesare all approximately the same, nor does the microscope distinguish anyapparent difference between those articles having the best physicalproperties and those which are inferior.

When manganese or other alloying metal having a high afiinity for oxygenis an ingredient of the starting material one aspect of thisinconsistency shows in the fact that there are sometimes oxideinclusions in the final product.

Final products showing such inclusions are definitely inferior in theirphysical characteristics, as illustrated by the following comparisons:

Two groups of steel compacts were made from apparently identicalstarting material which consisted of electrolytic iron powder, 0.55% ofa manganese-silicon eutectic manganese and 10% silicon) and 1% ofstearic acid lubricant. The final densities of the two groups weresubstantially the same. In one group there were numerous oxideinclusions in the final product while in the other the oxide inclusionswere few. The average physical properties of the first group were,tensile strength 122,000 p. s. i., elongation 15%, and reduction of area23%. The other group, which contained comparatively few oxideinclusions, had average physical properties of tensile strength 130,000p. s. i., elongation of 20%, and reduction of area of 36%.

Two other compacts made from starting material of apparently identicalcomposition and apparently processed in the same manner were tested forimpact strength. One compact had numerous oxide inclusions while theother had relatively few. The compact having relatively few inclusionshad an impact strength of 29 ft. lbs. while the one having numerousinclusions had comparatively an impact strength of only 14.6 ft. lbs.

One way of obtaining final products having relatively uniform highphysical properties and thus avoiding the above mentioned difficultiesis to remove the sorbed water and sorbed gases and to reduce any ironoxide contained in the iron powder used as a starting material orcomponentthereof, until the oxygen content of such iron powder, asdetermined by a vacuum fusion test, is reduced to substantially 0.2% orless. Instead. of a vacuum fusion test another test that may be made toshow whether the iron powder is sufficiently low in sorbed material isto heat the iron powder in dry hydrogen for 2 hours at 1800 F. anddetermine the loss in weight of the powder, during such heat treatment.Such loss in weight should not be substantially more than 0.2

It is found that removal of the sorbed material to the desired degreemay be efiected by heattreating the powder at elevated temperatures,such as 1350 F., in an atmosphere, such as hydrogen, for relatively longperiods of time, such as 3 hours, and thereafter re-pulverizing thesintered cake formed in such heat-treating step. However, that may be atoo costly operation.

Instead of processing the iron powder as above to remove sorbed materialI have found that articles can be obtained having consistently highphysical properties and which are consistently free from oxideinclusions by mixing carbon with iron powder having a high sorbedmaterial content, to form a starting material or component thereof,compressing such starting material to form a cake and thereaftersintering the cake in such a way as to remove a substantial amount ofthe carbon. During the sintering process carbon may be removed partly bycombination with oxygen, in whatever form the oxygen exists in thepressed cake, and partly by combination with a medium of the sinteringatmosphere, for example hydrogen, the gases thus formed escaping fromthe cake. In this way the oxygen escapes from the cake, in preference tocombining with or remaining combined with the iron or alloying agent. Itis thus removed instead of remaining in the cake as an oxide inclusion.

Where carbon has heretofore been included in the starting material forpowder metallurgy processes, it usually has been for the purpose ofproviding the entire carbon content of the desired final steelcomposition. Although this carbon may effectively remove substantiallyall of the oxygen, if the compact is heated, the large amount of carbonremaining is undesirable for processes which involve two or morepressing operations with intervening sinterings because this carbon,when dissolved in the iron during the first sintering operation, makesthe material too resistant to subsequent pressing or forming operations,thus resulting in a porous product. In this invention I remove asubstantial amount or even all of the carbon by the initial sinteringstep so that the pressed and sintered cake is soft and can be repressedto high density in a second pressing operation. Carbon can then beintroduced into the cake by any desired method, such as sintering thecake in a carburizing atmosphere, to produce a steel compact having muchhigher strength and ductility than would otherwise be possible.

In pressing the material I prefer to employ for my initial pressingoperation a pressure of less than 40 tons per square inch and for mysecond pressing operation a pressure of less than 100 tons per squareinch, preferably between 60 and 90 tons per square inch. In most cases Iwould like the minimum pressure used in the first pressing operation tobe substantially 15 tons per square inch. After my first pressingoperation I prefer that the cake have a density of 6.3-6.8 and thatafter the 2nd pressing operation the density be substantially 7.5-7.6 orover.

In my first sintering operation after the first pressing, I prefer touse an atmosphere of dry hydrogen or cracked anhydrous ammonia and toemploy a temperature of at least substantially 1800 F.

I prefer not to exceed 2050 F. when free carbon is an ingredient of thestarting material because at temperatures only slightly above this(approximately 2100 F.) melting occurs at the zones of contact betweenthe iron andthe free carbon in the compact due to the localizedformation at such contact zones of the iron-carbon eutectic alloy. Theexistence of this eutectic alloy is only temporary, being present onlywhile free carbon is still present. Normally, substantially all thiscarbon will be dissolved in the iron at the sintering temperature inabout 10 minutes or less. The presence of a liquid phase duringsintering is undesirable because it causes excessive shrinkage anddistortion.

I may use as starting material or as a component thereof iron powderwhich, aside from sorbed material, contains no more than substantially0.2% of other impurities. Said sorbed material may be present to theextent of 1%. The carbon component of the starting material should notexceed 0.6% and the carbon remaining after the first sintering operationshould not exceed 0.4%, and I prefer that these two amounts should notexceed substantially 0.3% and 0.1% respectively. As previously statedwhen a substantial amount of carbon is required in the final product Iprefer to introduce it after the second pressing operation.

While I have referred chiefly to manganese-as the alloying element, theinvention is also applicable to other elements having a strongeraflin-., ity for oxygen than has iron. These include such. elements asaluminum, chromium, vanadium, ti-.

tanium, silicon, tungsten, molybdenum, etc.

As an example of the practice of my invention I may take an electrolyticiron powder containing 0.6% to 0.8% oxygen, and mix it with 0.5% to.0.9% manganese metal powder or preferably suf-z ficient low-carbonferromanganese powder to beequivalent in manganese content, and with0.3% powdered graphite and about 1% of a lubricant such as stearic acid.I press this mixture at about: 15-40 tons per square inch to form acoherent;

compact and sinter the compact in dry hydrogen at 2000 F. until theremaining carbon does notexceed substantially 0.1%. This requires, inthe case of pieces 4" thick, approximately three hours. I then repressor coin the piece, using at least 75 tons per square inch, to finaldimensions.

The density of the compact will now'be approxistill reach densities of7.65 and over by employing a coining pressure of less than tons persquare inch if no metallic alloying ingredients are present. With 0.9%manganese added densities of 7.5 and over can be obtained at coiningpressures of less than 100 tons per square inch.

It is a feature of my invention that I have discovered that I need notuse as much carbon as is required for chemical combination with all of IFor example,. if I use an iron powder containing 06-08% of the sorbedoxygen in the powder.

oxygen, I find that if I add 0.3% carbon the oxygen will be completelyeliminated with the consumption of only 0.2% carbon, with 0.1% carbonremaining in the iron. Since it would require 0.45 to 0.6% carbon tocombine with the O.6-0.8%

oxygen if the reaction were simply a burning of carbon and oxygen tocarbon monoxide, I accom-f: plish my deoxidation of the powder with theconsumption of only approximately to /5- of the expected amount ofcarbon. This is an important feature of my invention because theinclusion of amounts of carbon such as 0.45 to 0.6%. in the startingpowder causes the powder compacts to be weak and frangible prior to thesintering operation.

After the coining or second pressing operaacsegsss tion, severalalternative procedures are possible. As examples, I will describe two:

- (1 If it is desired to produce a product which may be suitable forsubsequent machining operations I heat-treat the compact about 1 hr. ata temperature between 1000 F. and 1200 F. in a non-oxidizing atmospheresuch as dry hydrogen. This produces a structure of good machinabilitywhich is well adapted to conventional machining operations. Thereafterthe article may be subjected to any of the well-known surfacetreatments, such as case-hardening, plating, nitriding, Parkerizing,etc.

i (2) If it is desired to produce a steel product I may heat treat therepressed article in a carbur izing medium, preferably in a two-stagemethod such as disclosed and claimed in the co-pending application ofLyman F. Whitney, Serial No. 666,034, filed on even date herewith. Astherein more fully disclosed that method comprises using a richcarburizing medium and then changing the medium to reduce itscarburizing effect, thereby to equalize the distribution of carbon inthe article. The degree of richness of the initial medium may be variedto suit different conditions but it is always greater than theequilibrium concentration, that is the concentration of carbon in themedium at which the medium would be in substantial equilibrium with thefinished article. The change in richness may be made abruptly, as byreplacing a rich atmosphere by a lean atmosphere, or it may be madegradually, as by drawing off rich atmosphere and feeding in leanatmosphere. The change may he carried to or below the equilibrium point.Indeed the richness may be reduced to zero, as by replacing thecarburizing atmosphere with an inert atmosphere. If the richness iscarried below the equilibrium point it may be brought back to this pointeither gradually or abruptly. In equalizing the distribution of carbonin the article by changing the medium, the equalization may be carriedas far as desired and throughout all or only part of the article. Forexample, in many cases it is necessary to carry the equalization only tothe point where the distribution is partly equalized and only throughouta limited depth adjacent the surface of the article. Preferably theequalization of carbon in the article is effected by heattreating thearticle at approximately 1700" F. in an atmosphere of hydrogencontaining 0.30% of methane for a period of time which is determined bythe dimensions of the piece. The piece will have very nearly thecharacteristics of ordinary steel of similar carbon content and may befurther hardened by conventional quen hing procedures. While I havereferred to 1700" F. as my preferred carburizing temperature I do notlimit myself to this temperature as the temperature employed for thesetwo operations may range from 1600 F. to 2050 P. which is just below themelting point of the iron-carbon eutectic.

As a specific example of the utilization of my invention I have producedpieces of square cross section A" x and 2" long having final uniformcarbon content of 0.8%. Annealed electrolytic iron powder, 100 mesh,containing approximately 0.7% oxygen was mixed with 1% stearic acidlubricant, 0.63% 400 mesh low-carbon ferromanganese powder, and 0.3%graphite. The mixture was given an initial pressing at 2'7 tons persquare inch, followed by a pre-sintering heat treatment in hydrogen atabout 900 F. for about one hour to eliminate the stearic acid, and by aSintering for 3 hours at 2000 F. in dry hydrogen.

The compact was then given a final pressing at 90 tons per square inch.The compact was then carburized for 10 hours at 1700 F. in dry hydro-;gen containing 1.0% propane. It was then equalized for 22 hours at 1700F. in hydrogen con-. taining 0.30% methane. After cooling from this.heat treatment the piece was re-heated to 500 F., oil quenched, anddrawn at 1100 F. Its physical properties were then found to be: tensilestrength 131.400 lbs. per square inch. elongation 18.7%, reduction ofarea 36%, and Rockwell B hardness 99.

Other gases may be substituted for hydrogen if desired. Gases which Ican successfully use include cracked anhydrous ammonia, the conven-.tional partially combusted types'of heat-treating atmospheres,completely burned atmospheres of the type consisting of CO, N2, and H2in various proportions, and. inert gases such as nitrogen. Where methaneor other hydrocarbon is required to be used with any of the above gasesduring the: equalizing cycle, the proportion of methane or otherhydrocarbon may be so chosen and maintained as to be in equilibrium withthe carbon of the final desired steel composition.

- Carbon may be introduced into the piece in ways other than thosedescribed above. For example, instead of gaseous carburizingatmospheres, I may use a pack, or I may use liquid car-' burizing saltbaths. For an equalizing medium, I may substitute inert liquids such aslead or inert salt baths. When the compact is of high density, forexample 7.65-7.70, the pores in the compact are not interconnecting andthe liquids do not penetrate to the interior of the compact.

While I have referred to the use of a mixture of pure iron powder andone or more of my preferred alloying agents as a preferred startingmaterial or as one component thereof, I could equally well use an ironpowder in which one or more such alloying agents is dissolved, butcontaining aside from oxygen and other sorbed gases other impurities tothe extent of not over 0.2%.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents which fall within the scope of the appended claims.

I claim:

1. A method of making a soft iron compact substantially free of oxideinclusions comprising the steps of loosely mixing powdered ironcontaining non-ferrous solid impurities in an amount not exceeding 0.2%and sorbed materials including oxygen not exceeding 1%, with freecarbon, pressing the mixture at less than approximately 40 tons persquare inch to form a coherent compact, sintering said compact in anon-oxidizing atmosphere at the temperature at which the carbon and saidsorbed oxygen will combine in such proportion as to remove substantiallyall of the sorbed oxygen from the compact and to leave an excess ofcarbon in an amount not exceeding 0.4%, all percentages being by weight,and cooling said compact in a protective atmosphere to a temperatureinhibiting oxidation of the substantially pure iron of the compact.

2. A method of making a soft iron compact substantially free of oxideinclusions comprising the steps of loosely mixing substantially purepowdered iron containing non-ferrous solid impurities in an amount notexceeding 02% and sorbed material including oxygen not exceeding 1%,with manganese in an amount not exceeding 0.9% and free carbon, pressingthe mixture at less than approximately 40 tons per square inch to form acoherent compact, sintering said compact in a non-oxidizing atmosphereand at a temperature of not less than approximately 1800 F. and belowits melting point at which the carbon and said sorbed oxygen willcombine in such proportion as to remove from the compact substantiallyall of the sorbed oxygen, to cause the manganese to diffuse into theiron and to leave an excess of carbon in an amount not exceeding 0.4%,all percentages being by weight, and cooling said compact in aprotective atmosphere to a temperature inhibiting oxidation of thecompact.

3. A method of making metal articles having a high final density,comprising the steps of loosely mixing substantially pure powdered ironcontaining non-ferrous solid impurities in an amount not exceeding 0.2%and sorbed materials including oxygen not exceeding 1%, with freecarbon, pressing the mixture at a pressure of less than approximately 40tons per square inch to form a coherent compact, sintering said compactin a non-oxidizing atmosphere at the temperature at which said sorbedoxygen and carbon will comblue in such proportion as to remove from thecompact substantially all of the sorbed oxygen and leave carbon in anamount not exceeding 0.4%, all percentages being by Weight, cooling thecompact in a protective atmosphere to a temperature inhibiting oxidationof the iron and hence to provide a soft iron compact of intermediatedensity, and repressing said soft oxygenfree iron compact at a pressureof at least approximately sixty tons per square inch.

4. A method of making a steel article having a density above 7.5 andwhich is substantially free of oxide inclusions, comprising the steps ofloosely mixing powdered iron containing non-ferrous solid impurities inan amount not exceeding 0.2% and sorbed materials including oxygen notexceeding 1%, with manganese up to 0.9% and free carbon up to 0.6%,compacting the mixture un- REFERENCES CITED The following references areof record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,315,302 Volterra Mar. 30, 19432,333,573 Kalischer Nov. 2, 1943 2,362,007 Hensel et a1. Nov. 7, 19442,367,358 Kott et a1 Jan. 16, 1945 2,386,604 Goetzel Oct. 9, 19452,411,073 Whitney Nov. 12, 1946 Rice Feb. 3, 1948

